The ImageMagick suite is capable of all types of conversion. Using it is simple: convert originalfile.format newfile.newformat convert automatically determines the original filetype from the file itself and chooses the output format based on the file extension you provide for the newfile. For example, convert file.ps file.gif will convert the file from postscript to GIF. see the convert(1) man page for more information on the formats it supports.
While ImageMagick supports multi-page GIF files, many utilities and viewers do not, so you may wish to either convert single pages of the PS file to GIF format, or first use the psnup command to reduce (the relevant pages of) your document to a single page